Generally, electrophotographic toners are roughly divided into pulverized toners and polymerized toners. Pulverized toners are obtained by kneading components such as a colorant, a charge control agent and a releasing agent into a polymerized binder resin, then grinding and classifying the kneaded product. On the other hand, polymerized toners are obtained as colored polymer particles by polymerizing a mixture containing constituents such as a polymerizable monomer, a colorant, a charge control agent and a releasing agent using suspension polymerization, emulsion polymerization, dispersion polymerization or the like, then if needed agglomerating or aggregating.
For either of these techniques it is difficult to completely react the polymerizable monomer during the polymerization step, causing trace amount of unreacted polymerizable monomer to remain in the toner. If toner containing residual polymerizable monomers is used in an electrostatic imaging apparatus, residual polymerizable monomers are vaporized out of the toner by heating in the image fixing step, for example. This worsens the working environment and emits an offensive odor. Further, electrophotographic toners which contain a large amount of volatile substances like residual polymerizable monomers cause problems such as blocking during storage, offsetting during image fixing or filming on an electrostatic imaging apparatus members.
In case of pulverized toner, volatile substances such as residual polymerizable monomers can be easily removed during the binder resin production step or removed by heating to a high temperature during mixing and kneading of the binder resin, colorant, charge control agent and releasing agent.
For polymerized toners it is also necessary to remove volatile substances from the colored polymer particles that contain additive constituents such as a colorant, a charge control agent and a releasing agent after the polymerization step. However, unreacted polymerizable monomers are easily adsorbed on these additive constituents, making it difficult to decrease the amount thereof. Among polymerized toners, it is especially difficult to decrease volatile substances in low temperature fixable toners, because the toner tends to agglomerate when an attempt is made to remove the volatile substances such as the residual polymerizable monomer by heating at high temperature.
After the polymerization step, the dispersion comprising the colored polymer particles in polymerized toners is generally subjected to washing, dewatering and drying post treatment. To reduce the amount of volatile substances, such as residual polymerizable monomers, contained in polymerized toners, processes for removing volatile substances during these post treatment processes are known. Among such processes, treating the dispersion comprising the colored polymer particles before dewatering is being heavily investigated.
For example, Japanese Patent Application Laid-Open No. 5-66613 proposes a method which involves first suspension-polymerizing a polymerizable monomer in an aqueous medium, then distilling of the aqueous medium while it is being added to the suspension liquid. Although this publication discloses that a polymerized toner having a low amount of residual monomers can be obtained by using this method, it does not confirm to what extent residual monomers are actually removed from the polymerized toner. This method not only requires a large amount of aqueous medium, but the residual monomer elution rate from the polymerized toner particles is not very high. This publication goes on to disclose a method for distilling off an aqueous medium while heating at the peak top temperature or higher of the polymerized toner endothermic peak. Specifically, this publication discloses in Example 1 that water was distilled off while being added so that the amount of suspension liquid did not change while heating the suspension liquid at 150° C. under reduced pressure. These temperature conditions are well above water's boiling point, meaning that while distillating off of the water medium is possible, polymerized toner particles would tend to agglomerate or adhere to the apparatus, thereby decreasing productivity.
The present inventors proposed in Japanese Patent Application Laid-Open No. 2000-321809 a method which involves first subjecting to a low-pressure stripping process a dispersion of polymer particles obtained after a polymerization reaction has finished, which contains a colorant and a binder resin component, then drying. However, after further investigation it was found that under one of the processing conditions, the dispersion comprising the polymer particles foams in this low-pressure stripping process, whereby stable processing cannot be performed. It was also found that a large amount of time and heat is required in order to reduce the concentration of the residual polymerizable monomer to a predetermined concentration. For these reasons, a reduction method which is highly efficient and gives consideration to energy conservation has been desired.
Thus, while there have previously been various investigations into reducing the amount of volatile substances, such as residual polymerizable monomers, contained in a polymerized toner by treatment after polymerization, an adequate reduction method which is efficient and gives consideration to energy conservation is yet to be found.